Memory devices for non-volatile storage of information are in widespread use in a myriad of applications today. In general, as illustrated in FIG. 1, memory devices contain a memory array 102 of memory cells for storing data, a row decoder 104, and a column decoder 106 coupled to the memory array 102 for accessing the memory array 102 in response to an external address. Generally, drain regions of memory cells located in a column are connected to a common bit line (BL) 108 and control gates of memory cells in a row are connected to a common word line (WL) 110. Examples of non-volatile memory cells include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and flash EEPROM.
FIG. 2 illustrates a sectional view of one type of a flash memory cell. As shown in FIG. 2, the flash memory cell includes an n-type source region 204 and an n-type drain region 206 formed in a p-type silicon substrate 202. The region between the source region 204 and the drain region 206 is known as a channel region 208 of the memory cell. A dielectric layer including a top oxide layer 210, a nitride layer 212 and a bottom oxide layer 214 are formed above the channel region 208. A conductive layer 216 functioning as a control gate is formed over the dielectric layer.
Typically, operations for accessing data stored in the flash memory cell include programming, reading and/or erasing operations. Programming operations (i.e., charge injection) on a flash memory cell may be carried out by various methods, such as channel hot electron (CHE) injection or Fowler-Nordheim (FN) injection, both of which inject electrons into the nitride layer 212. Erasing operations (i.e., charge removal) may be accomplished by band-to-band hot hole tunneling (BTBHHT) or Fowler-Nordheim (FN) injection. To erase the memory cell, the electrons previously trapped in the nitride layer 212 may be neutralized either by moving electrons out of the nitride layer 212 or by transferring holes into the nitride layer 212.